It might not be long before the Padres officially shut down two of their starting outfielders for the rest of the season. Both players are dealing with problematic knees.

For the second time in a week, Cameron Maybin’s rehab assignment with Triple-A Tucson has been put on hold. The center fielder is back in San Diego, where he had his right knee re-evaluated Monday by the Padres’ team doctors.

Meanwhile, Carlos Quentin’s strained right knee hasn’t shown significant improvement since the left fielder received the third of three lubricant injections last Thursday.

Maybin has been on the disabled list since June 9 with a strain of his posterior cruciate ligament.

He previously had his rehab stint halted last Monday because of soreness in his knee. He has appeared in just one game since, last Friday as a designated hitter.

“He hasn’t quite been able to get over the hump, so there’s a chance he might get another opinion on his knee,” Padres manager Bud Black said. “He hasn’t gotten to the point in competition where he was able to take it up to that 100 percent level. The stability’s just not quite back yet.”

Essentially sharing Maybin’s pain, Quentin has also battled soreness in his knee since landing on the DL on Aug. 10. The Padres last Friday prescribed a period of rest not to exceed 10 days before re-evaluating his status.

“He’s not making the progress that we hoped for,” Black said Monday. “The knee’s not letting him do the things he needs to do to get back on the field.”

Asked about a return this season, Black said, “Right now it’s, I’d say, 50-50. I think next week we’ll have a better handle on that.”

Quentin, 30, is making $9.5 million this season. He is hitting .275 with 13 home runs and 44 RBIs in 82 games. He is owed at least another $17.5 million on a contract that runs through 2015 and includes a no-trade clause.

Maybin, 26, is making $3 million this season. He is hitting .157 (15-for-51) in just 14 games. He is owed at least another $21 million on a deal that runs through 2016.

The move cleared a spot on the 40-man roster, suggesting the Padres could promote a prospect before the end of the season. Black hinted such a move would not happen before Sept. 1, when major-league rosters expand.

Boxberger, 25, is beginning his fourth stint with the Padres this season. In six appearances spanning nine innings, he is 0-1 with a save and a 4.00 ERA.

“He’s shown a lot of flashes of great pitching, and there’s been some inconsistency too,” Black said. “We’re looking for the consistent, fundamental pitching principles from Brad, getting ahead in the count, putting guys away. He’s got big-league stuff.

“Hopefully this is the time he comes up and never goes back.”

O’Sullivan went 0-2 with a 3.96 ERA in seven games, including three starts, for the Padres this season. The 25-year-old Valhalla High graduate was signed to a minor-league contract last offseason. After going 7-4 with a 3.88 ERA with Tucson, he had his contract selected July 12.

“He pitched his butt off in Tucson, probably their best overall starting pitcher there for a long time,” Black said. “He came up and did some nice work for us.

“He was on board, a native San Diegan who grew up a Padres fan, nice guy, good on the team, all those things. So it was tough. But we just felt at this point we made a baseball decision.”

Blanks nearing rehab assignment

With Maybin and Quentin slowed in their recoveries, the news on a third Padres outfielder was more promising. Monday, Kyle Blanks took batting practice, ran in the outfield and participated in outfield drills.

Blanks, who has been on the DL since July 12 with Achilles tendinitis, is scheduled to begin a rehab assignment with Tucson “hopefully, best-case scenario, this weekend,” Black said.

Tucson’s Triple-A regular season ends Sept. 2.

Player of the week

Will Venable was named National League Player of the Week for the week of Aug. 12-18. The 30-year-old outfielder hit .406 (13-for-32) with two doubles, one triple, two home runs, five RBI, seven runs scored and a stolen base.